A micro-resonator is a single, uninterrupted, integrated optical waveguide element in the form of a closed loop that supports at least one resonant mode. The resonance wavelength(s) of the resonator is (are) a function of its optical length (i.e., path length and refractive index). The loop can be circular, “race track” (i.e., oval), elliptical, or even have an arbitrarily curved circumference.
The micro-resonator, which is often called a “ring resonator,” is typically used in conjunction with one or more linear waveguide regions. For example, FIG. 1 depicts a conventional four-port micro-resonator system (integrated micro-resonator”) 100 including loop waveguide 102 and two linear or “bus” waveguides 104 and 106. Bus waveguide 104 includes input port 104IP and through port 104TP. Bus waveguide 106 includes drop port 106DP and add port 106AP. A portion of each of bus waveguides 104 and 106 is adjacent and tangential to loop waveguide 102.
As a consequence of geometrical and other considerations, a portion of “on-resonance” light (light having the same wavelength as the one or more resonant wavelengths of the resonator) that is traveling along either of the bus waveguides (e.g., waveguide 104) couples to loop waveguide 102. A portion of that coupled light is, in turn, coupled to the other of the bus waveguides (e.g., waveguide 106). Light within the linear waveguides that is off-resonance (i.e., not at a resonance wavelength) bypasses the loop with only a small transmission loss (in theory, there is no loss).
As an example of the operation of conventional four-port micro-resonator 100, a multi-wavelength optical signal λX1 to X10 is launched into input port 104IP. All wavelengths of the signal remain in bus waveguide 104 and travel to through port 104TP except for wavelength(s) that are on-resonance with the loop 102. In this example, loop 102 is assumed to have a resonance at wavelength X7. As a consequence, light having wavelength X7 in the multi-wavelength optical signal couples to loop 102 and then to bus waveguide 106 traveling in the direction indicated toward drop port 106DP. In this fashion, the information contained in the signal at wavelength X7 is “dropped” from bus waveguide 104. Those skilled in the art will appreciate that the foregoing explanation is a simplification of the operation of the resonator (e.g., light makes multiple round trips around the loop before exiting, so there is interference within the loop, etc.).
Add port 106AP can be used to incorporate new data in the multi-wavelength signal that is traveling along bus waveguide 104. For example, in FIG. 1, a new signal λ′X7, which has wavelength X7, is launched into add port 106AP of bus waveguide 106. Since signal λ′X7 is on-resonance (with loop 102), it will coupled to loop 102 and then to bus waveguide 104 traveling in the direction of the original multi-wavelength single. As a consequence, multi-wavelength optical signal λX1 to X6, X8 to X10 and optical signal λ′X7 proceed to through-port through port 104TP.
The integrated micro-resonator can, therefore, effectively function as a highly-selective wavelength-dependent optical coupler to form optical “components” such as filters, routers, switches, and the like.
There are primarily three requirements for proper functioning of a micro-resonator: (1) a resonance condition, (2) a phase-matching condition, and (3) an evanescent field-coupling condition. These are explained further below.
Condition 1. The resonance condition results from constructive interference of light based on the optical round trip length (“ORTL”) of the closed loop.
Condition 2. The coupling of light between the loop and straight waveguides occurs within a “coupling length” (the length of the optical path over which coupling takes place). With continuing reference to FIG. 1, on-resonance light (i.e., light at wavelength X7) that is traveling in waveguide 104 couples, over optical path length CLB, to loop 102 over optical path length CLR. Over the arc angle θ, the coupled light will be out of phase due to the difference in optical path length: ΔOPL=CLB−CLR. It will be appreciated to those skilled in the art that the foregoing explanation of “phase mismatch” is necessarily simplified. A more rigorous treatment would involve the angle of the phase fronts of the propagating optical modes in loop 102 and bus 104 and other considerations.
Condition 3. In implementations such as integrated micro-resonator 100, light energy couples into and out of the loop waveguide via evanescent field coupling. An evanescent optical field is the portion of the optical field of guided light that extends beyond the physical surface of a waveguide. There must be enough overlap between the modes in waveguides at the coupling region in order for coupling to occur. Since the evanescent field does not extend far, loop waveguide 102 must be placed in close proximity to the linear waveguides for coupling.
The aforementioned three requirements lead to certain conventional wisdom and practice concerning micro-resonators.
At telecom wavelengths (about 1550 nanometers), a certain free spectral range (“FSR”) is required and that dictates the ORTL required for resonance. In prior art designs, this has resulted in a trade-off between the required ORTL, minimum bend radius for the loop, and acceptable bend loss. Ultimately, this trade-off favors making the geometrical round trip length (“GRTL”) as small as possible at telecom wavelengths. As a consequence, most loops that operate at telecom wavelengths are truly circular; that is, not a “race track” or other geometry.
This predilection for small GRTL at telecom wavelengths has typically resulted in integrated micro-resonators that use straight bus waveguides or, less frequently, waveguides that bend in the direction of the loop in order to increase coupling efficiency. This leads to the phase mismatch discussed above. To correct the phase-mismatch, it is known to adjust the loop waveguide (e.g., alter width and/or height, etc.) relative to the bus waveguides. This results in a change in the velocity of light through the loop, which effectively adjusts for the path length difference ΔOPL. But making such an adjustment necessarily causes an asymmetry of another property (e.g., width, height, etc.) between the bus waveguides and the loop.
The coupling process is intrinsically wavelength dependent, because for a given GRTL, the optical coupling length depends on wavelength and coupling factor (i.e., the fraction of optical power of a signal that is transferred between the loop and adjacent waveguide). The adjustment to correct for path length described above simply exacerbates the wavelength dependence.
At telecom wavelengths (about 1550 nanometers), wavelength dependence is not a significant issue; the primary focus is to achieve a round-trip length that is acceptably small. In US 2004/0008948, for example, an oval resonance device is disclosed that is intended to address the problem of phase mismatch. According to the reference, the oval shape of the resonator provides straight sides that are disposed adjacent and parallel to the linear input and output waveguides. Coupling occurs mainly in this straight portion of the oval resonator, such that the path-length differences and, hence, phase-mismatch as described above are reduced. Furthermore, according to the reference, the coupling length can be readily changed (by altering the length of the straight portion of the oval resonator). This enables oval resonators with the same width to operate with different coupling factors, as desired.
Of course, some coupling will occur beyond the straight portions, such that there will be some level of phase mismatch. Also, this solution to the problem of phase mismatch requires, of course, that the resonator be in the shape of an oval or “race-track.” Perhaps most significantly, at least in the context of telecom wavelengths, the race-track geometry effectively lengthens RTL (unless a material system is used that enables minimum bending radii that are suitably small). As previously discussed, the requirements for proper resonator operation prompt a small GRTL, which dictates a circular resonator, not an oval.
At lower-than-telecom wavelengths, such as datacom wavelengths (about 850 nanometers) and sensor wavelengths (typically between 405 to 850 nanometers), the wavelength dependence of the coupling factor becomes an important consideration. In fact, the present inventors found that micro-resonator devices they produced for operation at 850 nanometers operation were not functioning properly.
As a consequence, a need exists for a way to optically couple an optically-resonant waveguiding element to another optical waveguiding element that avoids or decreases phase mismatch issues while, at the same time, avoids or mitigates the wavelength-dependent behavior of the coupling factor.